This invention is directed to a method for the preparation of anti-static films on glass surfaces. More particularly, this invention relates to a method of manufacturing anti-static cathode ray tubes wherein adhesion of airborne dirt and unpleasant static discharges on the external surface of the face plate of the tube due to accumulation of static electricity are prevented by subjecting the face plate to anti-static treatment.
In recent years, due to the increased size, improved brightness and focus characteristics of cathode ray tubes, the voltage applied to the fluorescent surface of the tube, i.e. the acceleration voltage of the electron beam, is higher than it was in the earlier devices. In a conventional 21" color cathode ray tube, for example, this voltage was of the order of 25-27 KV, however in recent color tubes of 30" or larger size, the voltage is as high as 30-34 KV. The external surface of the face plate of the tube is therefore liable to suffer a charge build-up, especially when the television set is turned ON and OFF. As a result of this charge, fine airborne dirt adheres to the face plate. The dirt is easily noticed, and leads to a deterioration of the brightness of the color cathode ray tube. Further, when viewers approach the face plate of the tube, there is a discharge of static electricity from the face plate which is very uncomfortable.
FIG. 6 is a graph showing the variation of surface potential of the face plate of a cathode ray tube. In the figure, L is the curve of potential variation when the set is switched ON, and L1 is the curve of potential variation when the set is switched OFF.
In recent years, cathode ray tubes receive an anti-static treatment whereby, to avoid this accumulation of charge on the external surface of the face plate, a smooth transparent conducting film is formed on the plate so that the charge can escape to ground.
FIG. 5 is a drawing which describes the principle of the anti-static treatment of the cathode ray tube. In the figure, 6 is the neck of the tube which contains the electron gun (not shown in the figure). 7 denotes deflection yokes, 13 is a funnel, 4 is the face plate and 5 is a high voltage button. The deflection yokes 7 are connected to the deflection power supply via a lead wire 7a, the electron gun is connected to the drive power supply via a lead wire 6a, and the high voltage button 5 is connected to a high voltage power source via a lead wire 5a.
In the cathode ray tube of the above construction, the electron beam emitted by the electron gun inside the neck 6 is deflected electromagnetically from outside the tube by the deflection yokes 7, and a high voltage is applied at the same time to the fluorescent surface on the inner surface of the face plate 4 by means of the high voltage button 5. The electron beam is therefore accelerated, and its energy excites the fluorescent surface to cause emission of light. However, due to the high voltage applied to the fluorescent surface, the potential of the external surface of the face plate 4 changes, and dirt adheres to the plate.
To avoid adhesion of dirt, therefore, as shown in FIG 5, a smooth transparent conducting film 11 is formed on the face plate 4. As the film 11 is grounded, any charges on the plate escape continually to ground, and accumulation of charge on the plate is prevented.
In the anti-static cathode ray tube 3 shown in FIG. 5, the grounding of the transparent conducting film 11 on the external surface of face plate 4, is accomplished by electrically connecting the metal anti-implosion band 8 wound around the side wall of face plate 4 and the transparent conducting film 11 by means of a conducting tape 12. The anti-implosion band 8 is connected to ground 10A by means of a ground wire 10 attached to fixing lugs 9, and the transparent conducting film 11 may therefore be grounded easily.
Curves M and M1 in FIG. 6 respectively show the potential variation of the external surface of the face plate in such an anti-static cathode ray tube 3 where a smooth, transparent conducting film 11 has been formed on the surface, when the power to the tube is switched ON and OFF, respectively. It is seen from this figure that the charge build-up is much less than in tubes which have not been so treated.
The smooth, transparent conducting film 11 formed on the surface of the face plate 4 must have a certain degree of hardness and adhesion, and silica (SiO.sub.2)-type films are therefore generally used.
In the past, these silica-type smooth, transparent conducting films were formed by coating the face plate 4 uniformly and evenly with an alcohol solution of Si (silicon) alkoxides with --OH, --OR or other functional groups, for example by spin-coating. The films were then baked at a relatively low temperature, for example 100.degree. C. or below.
The smooth, transparent conducting films 11 formed by the above method are porous and, as they contain silanol groups (.tbd.Si--OH), their surface resistance is reduced by adsorption of airborne moisture. However, when these conventional films were baked at high temperature, the --OH of the silanol group was lost together with the moisture included in the pores. As a result, the surface resistance increased, and the desired conductivity could not be obtained. For this reason, baking must be carried out at low temperature, but in this case the film is not so strong. Further, if the films are used for long periods in a dry atmosphere, the moisture again escapes from the pores and the surface resistance increases with time. Moreover, once moisture has been lost from the pores, it does not easily re-enter them.
Conventional films thus suffered from the major disadvantages of poor strength and poor stability of electrical resistance with time. In order to overcome these drawbacks, metal atoms such as Zr (zirconium) were made to combine with the structure of the alkoxide in the coating solution so as to confer better conductivity, but this did not produce any great improvement.
The basic method of resolving these problems was to disperse a conducting filler of small particles of SnO.sub.2 (stannic oxide) or In.sub.2 O.sub.3 (indium oxide) in the alcohol solution of an Si (silicon) alkoxide, and adding minute quantities of P (phosphorus) or Sb (antimony) to the coating solution so as to confer semiconducting properties. By spin-coating the external surface of the face plate 4 of the cathode ray tube uniformly and evenly with such a solution, and then baking it at a fairly high temperature (e.g. 100.degree. C. -200.degree. C.), it was possible to increase the film strength, and to obtain a smooth, transparent conducting film 11 whose resistance did not change with time under any environmental conditions.
However, although an SiO.sub.2 (silica) film made by dispersing a conducting filler in an alcohol solution of an Si (silicon) alkoxide as described above, does have the above advantages, it still has a major problem in its characteristics as will be discussed below.
The external surface of the face plate 4 of a cathode ray tube was coated with an alcohol solution of an Si (silicon) alkoxide to which minute particles of SnO.sub.2 (stannic oxide) had been added in a proportion of 1.5 parts by weight with respect to the total weight of solution, and the coating was baked at 150.degree. C. for 30 minutes to produce anti-static cathode ray tubes. When various tests were performed on the anti-static cathode ray tubes so obtained, it was found that the surface resistance was 5.times.10.sup.6 .OMEGA., the film strength was no less than 9H on the scale of lead pencil hardness, the surface resistance under dry conditions showed no variation at all, and the charge build-up when the television set was switched ON and OFF very closely resembled the characteristics shown by M and M1 in FIG. 6. It was however noticed that when the back of the hand was moved across the surface of the smooth, transparent conducting film 11 when the set was ON, a slight vibration was felt. This vibration was not the kind of shock felt when the face plate was charged. It was found to be a problem peculiar to silica (SiO.sub.2) films in which a conducting filler was dispersed, and did not occur at all with conventional cathode ray tubes. Moreover, to some people, it was felt very uncomfortable.
The cause of this vibratory feeling was examined. It was found that when the coating was deposited by a wet process such as spin-coating an alcohol solution of an Si (silicon) alkoxide containing a dispersion of conducting filler particles, and the amount of filler increases, the particles cohere together rapidly when the coating dries. Viewed microscopically, as shown in FIG. 4, the filler particles 2 form a chain-like meshwork in an SiO.sub.2 (silica) matrix 1. Viewed macroscopically, the smooth, transparent conducting film 11 has no charge build-up as the charge escapes to ground. Viewed microscopically, however, after the television set is switched ON, the potential on the surface of film 11 remains unequally distributed over the meshwork even after a considerable time has elapsed. If, therefore, the back of the hand is moved across the surface, a vibration is felt as if the back of the hand is shaking.
The spin-coating or other coating process which was used in view of their efficiency of production and easiness in handling of the cathode ray tubes, was carried out after a metal anti-implosion band 8 had been wrapped around the side wall of the face plate 4 of the tube.
FIG. 7A is a schematic diagram of the process steps used to manufacture the conventional cathode ray tube. In the figure, 20 is panel-mask assembly, 21 is panel-mask pair-bake, 22 is coat-deposition and aluminum deposition, 23 is panel bake, 24 is frittered glass sealing, 25 is gun sealing, 26 is evacuation, 27 is seasoning and aging, 28 is characteristics testing, 29 is anti-implosion treatment, and 30 is shipment. Cathode ray tubes are manufactured by process steps 20-30 in stated order.
FIG. 7B is a schematic diagram of the process steps used to manufacture the conventional anti-static cathode ray tube. As can be seen from the figure, in the conventional method used to manufacture the anti-static cathode ray tube, an anti-static treatment 31 is carried out between anti-implosion treatment 29 and shipment 30 in the manufacturing sequence for conventional tubes shown in FIG. 7A. This anti-static treatment 31 consists of coating with a solution, e.g. by means of spin-coating 31A, and baking 31B. As the other process steps are the same as in FIG. 7A, they are given identical numbers and their description will be omitted.
In the anti-static cathode ray tube with a smooth, transparent conducting film described above, therefore, a conducting filler was added to improve the strength of the film and prevent its surface resistance from varying with time. However, when the film was applied to the face plate by a wet method such as spin-coating, the conducting filler particles formed a chain-like meshwork in an SiO.sub.2 (silica) matrix. As a result, when the television set was ON and the back of the hand was moved across the surface of the transparent conducting film, there was an unpleasant sensation as if the hand was shaking.
The above manufacturing method was also associated with two other problems insofar as concerned the manufacturing process and film performance.
Regarding the manufacturing process, the baking of the film requires a new furnace to be installed. The baking condition of 150.degree. C. must be maintained for 30 minutes, and the addition of this step demands that a furnace length of 50-100 m be allowed for continuous processing, although the actual required length differs depending on the capacity of the production line and the size of the cathode ray tube to be manufactured. The addition of this furnace to the production line was therefore a great disadvantage from the viewpoint of the space that is required.
Regarding film performance, the film had to be baked at a temperature no greater than 200.degree. C. once it had been formed on the finished cathode ray tube, so as not to adversely affect the reliability or lifetime of the tube. In the case of conventional anti-static tubes, however, the strength of the transparent conducting film was inadequate. For SiO.sub.2 (silica) films, the strength of the film increases with the baking temperature, and at temperatures of 350.degree. C. or over, it is almost the same as that of glass. Because of the above restrictions, however, film strength was insufficient. Moreover, applying a further heat treatment to the tube once it had been manufactured entailed considerable energy losses.
FIG. 11 shows the spin-coating process in the conventional method of manufacturing an anti-static cathode ray tube. Anti-implosion treatment is first carried out with the metal anti-implosion band. After the external surface of the face plate 104 has been cleaned, the funnel 113 of the tube is supported on the platform 118 of a spin-coating machine 114, and the holes of fixing lugs 9 are fixed on columns 117 such that face plate 104 points upwards. In this position, the tube is rotated at a relatively low speed (e.g. 40-60 rpm), and a certain quantity of coating solution 19 is sprayed onto the external surface of face plate 104 from an injection nozzle 116 above the face plate. Once the coating solution 119 has spread to some extent all over the external surface of the face plate 104, the speed of the spin-coating machine 114 is raised (e.g. to 100-150 rpm) so as to spin the cathode ray tube at high speed, thereby distributing the coating film evenly over the face plate and stabilizing it.
In the above spin coating process, coating solution which splashes off as the cathode ray tube rotates is caught by salvage cap 115.
There were however two problems in the conventional method of manufacturing anti-static cathode ray tubes.
The first problem was that irregularities occured due to uneveness of the coating in one pair of diagonal corners of the face plate 104. In order to make the coating uniform and stabilize it, the cathode ray tube is rotated at high speed. As FIG. 9 shows, however, the rectangular face plate 104 stirs up the air inside the circular salvage cap 115, and further, the degree of air turbulence is different along the long and short sides of the rectangle. As a result, when the tube is rotated in the direction shown by the arrow in FIG. 9, corner irregularities 122 appear due to uneveness of the coating in the upper left and lower right corners. These corner irregularities 122, moreover, could not be elimated by adjusting the rotation speed of the tube or adjusting the viscosity of the coating solution.
The second problem was that drops of coating solution which had splashed off due to high speed rotation of the tube, impinged on the bottom wall 115A of salvage cap 115 from the same oblique angle as the direction of rotation, and then rebounded back toward the funnel 113 of the tube. Because of the air turbulence around the rotating tube, the movement of the drops of the coating solution is disturbed and the drops may adhere to the tube. Adhesion of drops is a serious problem particularly when the adhesion is in the vicinity of high voltage button 105, leading to high voltage leaks.